Wireless communication systems are widely deployed to provide various types of communication contents such as voice, video, data and the like. Transceiver devices for wireless communication systems typically comprise high speed analogue or digital front end (AFE, DFE) devices. These AFE/DFE devices typically comprise digital processors which are used to perform complex processing while adhering to reasonable power and size constraints. In order to transfer information to another radio device, the data signals are normally first converted into analogue signals which may then be sent by a suitable antenna. The conversion process is performed by digital-to-analogue converters.
In electronics, a digital-to-analogue converter (DAC, D/A, DA) is a device that converts a digital signal into an analogue signal. An analogue-to-digital converter (ADC) performs the reverse function. There are specific DAC architectures for different applications. For example, high-speed DACs are used to transmit at high data rates. These DAC types permit moving some of the traditional analogue functionality of a radio system into the digital domain, such as frequency up-conversion. One technique to improve the sampling rate of a DAC system is to use multiple DACs in parallel. A parallel DAC architecture typically includes multiple DACs whose outputs are combined in the analogue domain.
For high-speed DFE applications, a single DAC element uses an externally generated clock signal. This clock signal is used within the DAC to generate a carrier signal which is then modulated with the transmit data. However, one problem with this approach is a potential misalignment of the different output signals of the DAC elements of a multi DAC array with regard to their phase shift. As a consequence of this, the clock generators which are triggering the DAC elements have to be reset regularly and sometimes for each data sample in order to ensure a predefined phase shift. However, for high-speed applications, in particular for wireless RF data communication, this resetting is decreasing the data throughput significantly.
A predefined phase shift is in particular essential for beamforming. Beamforming is used for transmitting RF signals in order to generate a predefined beam form. With the beamforming technique, so-called smart multiple antennas are used. Smart antennas are arrays of antenna elements, wherein each of these antenna elements receive an analogue signal from a DAC. This analogue signals are to be transmitted with a predetermined phase offset and a relative gain. The net effect of the array is to direct a transmit beam in a predefined direction. The beam is steered by controlling the phase and gain relationships of the signals that excite the antenna elements of the array. However, in case, that the different transmit signals are not sent at a predefined point of time from the different antenna elements, it will be nearly impossible to obtain a predefined form of the beam.
U.S. Pat. No. 8,286,067 B2 describes a method for transmitting sampled data and control information between a digital signal processor and an RF analogue front end device. The analogue front end device comprises two DACs within the transmit path. The clock signal for triggering these DACs is generated externally. For a given clock rate, the serial interface transmitter of the analogue front end device provides variable data rates by inserting null frames to match the data rate to the clock rate. The null frames may be identified by specific codes within the control field. Thus, U.S. Pat. No. 8,286,067 B2 discloses a method for controlling multiple DACs within a modem, wherein the control data for controlling the DACs is included within the transmitted data.